The present invention relates to a multispectral photodiode for infrared radiation comprising a substrate having, a first semiconductor layer of a first conductivity type with a large band gap arranged thereon, a second semiconductor layer of the first conductivity type with a small band gap arranged on the first semiconductor layer, a first diode area, formed from a first zone of a second conductivity type with the first semiconductor layer, and a second diode area formed from a second zone of the second conductivity type with the second semiconductor layer.
A multispectral planar photodiode for infrared radiation, which is suitable for arrangement in an array, is known from U.S. Pat. No. 6,034,407. This known photodiode comprises an arrangement of the following semiconductor layers of a first conductivity type: a substrate, upon which is arranged a buffer layer, upon which is arranged a first active layer which is sensitive to a first wavelength range, upon which is arranged a barrier layer, upon which is arranged a second active layer which is sensitive to a second wavelength range, and upon which is arranged a cap layer. A first zone of second conductivity is embedded in the first active layer and forms with this a first diode area. A second zone of second conductivity is embedded in the second active layer and forms with this a second diode area. The zones of second conductivity are also covered by the cap layer, and are contacted through holes by a structured metal film. On the metal film, so-called indium bumps, with which the first and second diode areas can be connected to an external readout circuit, are arranged on corresponding raised points of the photodiode. In this known arrangement, the second diode area is arranged around the first diode area which lies in the center. Whereby the first diode area lies in a trench which extends from the surface through the barrier layer into the first active layer. The photodiode is irradiated from the substrate side. The first active layer has a large band gap, absorbs radiation with a shorter wavelength, and is transparent to longer wavelength radiation. The second active layer has a small band gap, absorbs radiation with the longer wavelength which has not been absorbed while passing through the first active layer.
In the range of infrared radiation at wavelengths of 3-5 xcexcm (MWIR) and 8-10 xcexcm (LWIR), the photon flow (at target temperatures around 300 K) quickly reduces as the wavelengths become shorter. However, the known multispectral, planar photodiode has the disadvantage that, because of the relations between the areas of the first and second diode areas, the first diode area, which is sensitive to the short-wave portion, is smaller than the second diode area, which is sensitive to the long-wave portion. However, at the same time, more photons are available in the long-wave range than in the short-wave range. This means that the diode area with a smaller area has to cope with a lower photon flow. This circumstance requires an increased computation effort in automated image processing, which is undesirable, particularly with time-critical applications.
The object of the invention is therefore to provide a multispectral photodiode, in which the relations of the areas of the first and second diode areas are matched to the relations of the photon flows of the assigned wavelength regions at the temperatures under consideration.
This object of the invention generally is solved according to the invention by a multispectral photodiode for infrared radiation comprising a substrate having a first semiconductor layer of a first conductivity type with a large band gap arranged thereon; a second semiconductor layer of the first conductivity type with a small band gap arranged on the first semiconductor layer; a first diode area, formed from a first zone of a second conductivity with the first semiconductor layer; and a second diode area, which is formed from a second zone of the second conductivity with the second semiconductor layer; and wherein the first diode area is arranged concentrically around the second diode area. Advantageous embodiments of the invention are achieved in accordance with the disclosed features of the invention.